Ac-dc power converter

ABSTRACT

In one embodiment, an AC-DC power converter can include: (i) a rectifier bridge and filter to convert an external AC voltage to a DC input voltage; (ii) a first energy storage element to store energy from the DC input voltage via a first current through a first conductive path when in a first operation mode; (iii) a second energy storage element configured to store energy from a second DC voltage via a second current through a second conductive path when in the first operation mode; (iv) a transistor configured to share the first and second conductive paths; (v) the first energy storage element releasing energy to a third energy storage element and a load through a third conductive path when in a second operation mode; and (vi) the second energy storage element releasing energy to the load through a fourth conductive path during the second operation mode.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No.201210538817.5, filed on Dec. 11, 2012, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of electronics, and moreparticularly to an AC-DC power converter.

BACKGROUND

An AC-DC power converter is used to convert an AC voltage into aconstant DC electrical signal, such as a DC voltage or DC current.Because of the relatively high power of AC-DC power converters, they arewidely used to drive high power loads (e.g., motors, light-emittingdiode [LED] lights, etc.). An AC-DC power converter can include arectifier bridge to convert the external AC voltage into a sinehalf-wave DC input voltage for the conversion circuit. To reduce AC gridharmonic pollution, an AC-DC power converter may utilize a power factorcorrection (PFC) circuit through which a relative high power factor canbe obtained.

SUMMARY

In one embodiment, an AC-DC power converter can include: (i) a rectifierbridge and filter configured to convert an external AC voltage to a sinehalf-wave DC input voltage; (ii) a first energy storage elementconfigured to store energy from the sine half-wave DC input voltage viaa first current through a first conductive path when in a firstoperation mode, where the first current rises during the first operationmode; (iii) a second energy storage element configured to store energyfrom a second DC voltage via a second current through a secondconductive path when in the first operation mode, where the secondcurrent rises during the first operation mode; (iv) a transistorconfigured to share the first and second conductive paths; (v) the firstenergy storage element being configured to release energy to a thirdenergy storage element and a load through a third conductive path whenin a second operation mode, where the second DC voltage is configured tobe generated on the third energy storage element, and where the firstcurrent declines during the second operation mode; and (vi) the secondenergy storage element being configured to release energy to the loadthrough a fourth conductive path during the second operation mode, wherea peak value of the first current is configured to vary along with thesine half-wave DC input voltage, and an output of the AC-DC converter isconfigured to be substantially constant.

Embodiments of the present invention can provide several advantages overconventional approaches, as may become readily apparent from thedetailed description of preferred embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an example single-stage AC-DCpower converter.

FIG. 2 is a schematic block diagram of an example two-stage AC-DC powerconverter.

FIG. 3A is a schematic block diagram of a first example AC-DC powerconverter in accordance with embodiments of the present invention.

FIG. 3B shows a conductive path diagram for the AC-DC power converter ofFIG. 3A in a first operation mode.

FIG. 3C shows a conductive path diagram for the AC-DC power converter ofFIG. 3A in a second operation mode.

FIG. 4A is a schematic block diagram of a second example AC-DC powerconverter in accordance with embodiments of the present invention.

FIG. 4B shows a conductive path diagram for the AC-DC power converter ofFIG. 4A in the first operation mode.

FIG. 4C shows a conductive path diagram for the AC-DC power converter ofFIG. 4A in the second operation mode.

FIG. 5A is a schematic block diagram of a third example AC-DC powerconverter in accordance with embodiments of the present invention.

FIG. 5B shows a conductive path diagram for the AC-DC power converter ofFIG. 5A in the first operation mode.

FIG. 5C shows a conductive path diagram for the AC-DC power converter ofFIG. 5A in the second operation mode.

FIG. 6A is a schematic block diagram of a fourth example AC-DC powerconverter in accordance with embodiments of the present invention.

FIG. 6B shows a conductive path diagram for the AC-DC power converter ofFIG. 6A in the first operation mode.

FIG. 6C shows a conductive path diagram for the AC-DC power converter ofFIG. 6A in the second operation mode.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention may be described in conjunction with thepreferred embodiments, it may be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it may be readilyapparent to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, processes, components, structures, and circuitshave not been described in detail so as not to unnecessarily obscureaspects of the present invention.

Referring now to FIG. 1, shown is an example single-stage AC-DC powerconverter. In this particular example, the single-stage AC-DC powerconverter can include single-stage power factor correction (PFC) maincircuit 10 and single-stage PFC control circuit 20. For example, thesingle-stage PFC main circuit may be a flyback topology, and thesingle-stage PFC control circuit can include current closed loop controlcircuit 21, current control circuit 22, flip-flop circuit 23, theisolation circuit, and multiplier U5.

Current closed loop control circuit 21 may sample the output current ofthe single-stage PFC main circuit. After flowing through the isolationcircuit, the output signal of circuit 21 can be provided along with theinput voltage to multiplier U5. Multiplier U5 can generate a signal thatacts as a reference signal for the in-phase input terminal of currentcontrol circuit 22. The inverted input terminal of circuit 22 can samplethe input current, and the output of current control circuit 22 can beprovided to zero trigger circuit 23. Zero trigger circuit 23 can includevoltage comparator U3 and RS flip-flop U4.

An output of current control circuit 22 and an output of voltagecomparator U3 can be coupled to reset R and set S of the RS flip-flop,respectively. The output of RS flip-flop U4 can essentially make theinput current vary along with the variation of the input voltage bycontrolling the state of switch S. In this way, the power factor of thesingle-stage PFC circuit may be improved relative to other approaches.However, when the output current has relatively large ripples (e.g., dueto a transient load), the error of the output current may also berelatively large. Therefore, the input current may have relatively largeerrors, and may not accurately vary along with the input voltagevariation, thus reducing the power factor.

Referring now to FIG. 2, shown is a schematic block diagram of anexample two-stage AC-DC power converter. In this particular example, theAC-DC power converter can include two-stage power stage circuits 203 and205, as well as a first-stage control circuit 204 and a second stagecontrol circuit 206. The first stage power stage circuit 203 can receivethe sine half-wave DC input voltage (e.g., V_(in)). First stage controlcircuit 204 can control first stage power-stage circuit 203 to make thewave of the input current vary along with the variation of the sinehalf-wave DC input voltage, so as to realize power factor correction.Second stage power stage circuit 205, which may be cascaded to the firststage power stage circuit, can receive output voltage V_(out1) of thefirst stage power stage circuit 203. According to driving voltagesrequired by light-emitting diode (LED) light 207, second stage controlcircuit 206 can control second stage power stage circuit 205 to providesubstantially constant output current and output voltage.

The example AC-DC power converter of FIG. 2 may have relatively goodoperational effects on harmonic waves, and can achieve a relatively highpower factor. This example power converter has an independent PFC stage,through which pre-adjustment can occur for the DC voltage provided to beDC-DC stage. Thus, the output voltage may be relatively accurate, andthis approach may be particularly suitable for high power applicationswith good on-load capacity. However, at least two sets of controlcircuits and power transistors are utilized in this approach, thusincreasing product costs. Further, the power density may be relativelylow, and the power consumption may be relatively large. Thus, thisconverter structure may not be particularly suitable for small or middlesized power electronic equipment.

In one embodiment, an AC-DC power converter can include: (i) a rectifierbridge and filter configured to convert an external AC voltage to a sinehalf-wave DC input voltage; (ii) a first energy storage elementconfigured to store energy from the sine half-wave DC input voltage viaa first current through a first conductive path when in a firstoperation mode, where the first current rises during the first operationmode; (iii) a second energy storage element configured to store energyfrom a second DC voltage via a second current through a secondconductive path when in the first operation mode, where the secondcurrent rises during the first operation mode; (iv) a transistorconfigured to share the first and second conductive paths; (v) the firstenergy storage element being configured to release energy to a thirdenergy storage element and a load through a third conductive path whenin a second operation mode, where the second DC voltage is configured tobe generated on the third energy storage element, and where the firstcurrent declines during the second operation mode; and (vi) the secondenergy storage element being configured to release energy to the loadthrough a fourth conductive path during the second operation mode, wherea peak value of the first current is configured to vary along with thesine half-wave DC input voltage, and an output of the AC-DC converter isconfigured to be substantially constant.

Referring now to FIG. 3A, shown is a schematic block diagram of a firstexample AC-DC power converter in accordance with embodiments of thepresent invention. In this example, after being rectified and filteredby rectifier bridge BR and filter capacitor C₁, the external AC voltagecan be converted into sine half-wave DC input voltage V_(in). The AC-DCpower converter can also include a first energy storage element (e.g.,inductor L₁), a second energy storage element (e.g., transformer T₁including primary side windings L_(p) and secondary side windingsL_(s)), and a third energy storage element (e.g., capacitor C₂). Inaddition, the AC-DC power converter can include control and drivingcircuit 301, which can control a switching state (e.g., on or off) oftransistor Q.

Referring now to FIG. 3B, shown is a conductive path diagram for theAC-DC power converter of FIG. 3A in a first operation mode. When in thefirst operation mode, control and driving circuit 301 can controltransistor Q to turn on, and inductor L₁, diode D₁, and switch Q canform a first conductive path (denoted by an encircled “1”). The sinehalf-wave DC input voltage can store energy in inductor L₁ by the firstconductive path, and current I₁ flowing through inductor L₁ can rise(e.g., continuously) as part of the first conductive path.

Also during the first operation mode, capacitor C₂, primary sidewindings L_(p) of transformer T₁, and transistor or switch Q can form asecond conductive path (denoted by an encircled “2”). In the secondconductive path, DC voltage V_(bus) across capacitor C₂ can releaseenergy to primary side windings L_(p). Transformer T₁ can store energy,and current I₂ of secondary side windings L_(p) can rise (e.g.,continuously) as part of the second conductive path.

Referring now to FIG. 3C, shown is a conductive path diagram for theAC-DC power converter of FIG. 3A in a second operation mode. When in thesecond operation mode, control and driving circuit 301 can turn offtransistor Q, and inductor L₁, diode D₁, primary side windings L_(p) oftransformer T₁, and capacitor C₂ can form a third conductive path(denoted by an encircled “3”). Inductor L₁ can release energy, andcurrent I₁ can decline (e.g., continuously) as part of the thirdconductive path. For example, a switching cycle of the AC-DC powerconverter can include the first and second operation modes.

A portion of the energy released by inductor L₁ can be transferred tothe load by transformer T₁, and a remaining portion of the energyreleased by inductor L₁ can be used to charge capacitor C₂. DC voltageV_(bus) can be generated across the two terminals of capacitor C₂.Secondary side windings L_(s) of transformer T₁, diode D₂, and capacitorC₃ can form a fourth conductive path (denoted by an encircled “4”). Theenergy stored in transformer T₁ can be transferred to the load throughthe fourth conductive path.

For example, diode D₁ may be used to prevent current of the thirdconductive path from flowing back to the input terminal in the secondoperation mode. In addition, control and driving circuit 301 can receivepeak current signals I_(pk2) and I_(pk2) of the first current I₁(flowing through inductor L₁) and the second current I₂ (of secondaryside windings L_(p)). Control and driving circuit 301 can also controltime t_(off1) and t_(off2). For example, t_(off1) is the time it takesfor the current of inductor L₁ to drop to zero from its peak value, andt_(off2) is the time it takes for the current of magnetizing inductanceof transformer T₁ to drop to zero from its peak value. By controllingt_(off1) and t_(off2), circuit 301 can generate a driving signal tocontrol the switching action of transistor Q, so as to realize powerfactor correction and a substantially constant output current. Forexample, peak current signal I_(pk1) and I_(pk2) can be obtained bysampling the first current I₁ and the second current I₂ by any suitablepeak value sampling circuitry.

In addition, the first and third conductive paths in this example canform a boost power stage circuit. The boost power stage circuit canreceive sine half-wave DC input voltage V_(in), and may generate asubstantially constant DC voltage V_(bus) across capacitor C₂. When avalue of capacitor C₂ is relatively large, the fluctuation of voltageV_(bus) across capacitor C₂ can be relatively small. Also, the secondand fourth conductive paths can form a flyback power stage circuit. Theflyback power stage circuit can receive V_(bus), and may generate asubstantially constant output voltage V_(o) by the fourth conductivepath, and a substantially constant output current I_(o) to drive theload (e.g., an LED load).

In the example of FIGS. 3A-3C, the first conductive path of the boostpower stage circuit and the second conductive path of the flyback powerstage circuit may share transistor Q and control and driving circuit301. Thus, transistor Q and control and driving circuit 301 can beutilized in both boost and flyback power stage topologies. As such, thestructure of this example AC-DC power converter may represent asimplified control structure, as compared to other approaches.

The following will describe power factor correction realization andsubstantially constant output signals, as well as the conductive pathsunder different operation modes, for this example AC-DC power converter.According to operating principles of a flyback power stage circuit, whenthe excitation inductance current of in the transformer works at aboundary conduction mode (BCM) and the time at which the current ofinductor L₁ drops to zero is earlier than the time at which theexcitation inductance current in transformer drops to zero, the outputcurrent can be calculated by the following formula (1).

$\begin{matrix}{I_{o} = {{I_{{pk}\; 1} \times \frac{n}{2} \times \frac{T_{{off}\; 1}}{T_{S}}} + {I_{{pk}\; 2} \times \frac{n}{2} \times \frac{T_{{off}\; 2}}{T_{S}}}}} & (1)\end{matrix}$

For example, I_(pk1) may denote the peak value of the first current ofinductor L₁, and I_(pk2) may denote the peak value of the second currentof the secondary side windings of transformer T₁. Also, n may denote aratio of the windings between primary side windings L_(p) and secondaryside windings L_(s) of transformer T₁. Further, t_(off1) can denote thetime it takes for the current of inductor L₁ to drop to zero from itspeak value, and t_(off2) can denote the time it takes for the current ofexcitation inductance of the transformer to drop to zero from its peakvalue. Also, t_(S) may denote a switching period (e.g., the sum oft_(on) and t_(off2)).

For example, peak value I_(pk1) of the first current can be obtained bythe following formula (1.1).

$\begin{matrix}{I_{{pk}\; 1} = {\frac{V_{in}}{L_{1}} \times t_{on}}} & (1.1)\end{matrix}$

Here, V_(in) may denote a sine half-wave DC input voltage, L₁ can denotean inductance value of inductor L₁, and t_(on) can denote a conductiontime of transistor Q. Peak value I_(pk2) of the second current can beobtained by the following formula (1.2).

$\begin{matrix}{I_{{pk}\; 2} = {\frac{V_{bus}}{L_{2}} \times t_{on}}} & (1.2)\end{matrix}$

Here, V_(bus) can denote the DC voltage across capacitor C₂, and L₂ candenote the inductance value of inductor L₂ (see, e.g., FIG. 4A). Inaddition, time t_(off1) for the current of inductor L₁ to drop to zerofrom its peak value can be obtained by the formula (1.3).

$\begin{matrix}{t_{{off}\; 1} = {\frac{V_{in}}{V_{bus} + {n\; V_{o}} - V_{in}} \times t_{on}}} & (1.3)\end{matrix}$

For example, V_(o) is the output voltage of the AC-DC power converter,such as provided at the load. Time t_(off2) may be the time it takes forthe current of magnetic inductance to drop to zero from its peak value,and its value can be obtained by the formula (1.4).

$\begin{matrix}{t_{{off}\; 2} = {\frac{V_{bus}}{n\; V_{o}} \times t_{on}}} & (1.4)\end{matrix}$

Switching period t_(S) can be indicated as below in formula (1.5).

$\begin{matrix}{t_{S} = {{t_{on} + t_{{off}\; 2}} = {\frac{V_{bus} + {n\; V_{o}}}{n\; V_{o}} \times t_{on}}}} & (1.5)\end{matrix}$

Rearranging formulas of I_(pk1), I_(pk2), t_(off1), t_(off2) and t_(S)into formula (1) of I_(o) can provide formula (2).

$\begin{matrix}{I_{o} = {\frac{n}{2\left( {{n\; V_{o}} + V_{bus}} \right)} \times t_{on} \times \left\lbrack {\frac{V_{in}^{2} \times n\; V_{o}}{\left( {V_{bus} + {n\; V_{o}} - V_{in}} \right) \times L_{1}} + \frac{V_{bus}^{2}}{L_{2}}} \right\rbrack}} & (2)\end{matrix}$

From formula (2), other than sine half-wave DC input voltage V_(in), allthe other voltages in the formula may be substantially constant values.Thus, in order to make I_(o) constant, conduction time t_(on) of thetransistor may be controlled to make the product of the conduction timet_(on) and the first polynomial of formula (2) a constant value.Conduction time t_(on) can be controlled by the control and drivingcircuit. In this example, control and driving circuit 301 can adjustconduction time t_(on) to control the output current I_(o) to besubstantially constant according to peak value I_(pk1) of the firstcurrent, peak value I_(pk2) of the second current, time t_(off1) for thecurrent of inductor L₁ to drop to zero from its peak value, and timet_(off2) for the current of primary side windings to drop to zero fromits peak value.

Control and driving circuit 301 can be implemented using any suitablecircuitry. As can be seen from the above control solutions, the controland driving circuit can sample the primary side signal and calculate theoutput current according to the sampled primary side signal. In thisfashion, substantially constant output current control can be realizedby way of primary side control. According to operating principles of theboost power stage circuit, input current I_(in) (the first current ofinductor L₁) can be calculated by formula (3).

$\begin{matrix}{I_{1} = {\frac{I_{{pk}\; 1}}{2} \times \frac{t_{on} + t_{{off}\; 1}}{t_{S}}}} & (3)\end{matrix}$

According to formula (2), conduction time t_(on) can be obtained asshown in formula (3.1).

$\begin{matrix}{t_{on} = {\frac{2 \times I_{o} \times \left( {{n\; V_{o}} + V_{bus}} \right)}{n} \times \frac{\left( {V_{bus} + {n\; V_{o}} - V_{in}} \right) \times L_{1} \times L_{2}}{{V_{in}^{2} \times n\; V_{o} \times L_{2}} + {V_{bus}^{2} \times L_{1} \times \left( {V_{bus} + {n\; V_{o}} - V_{in}} \right)}}}} & (3.1)\end{matrix}$

By rearranging the computational formulas of I_(pk1), t_(on), t_(off1),t_(S) and t_(on) into formula (3) formula (4) can be obtained.

$\begin{matrix}{I_{1} = {V_{in} \times \frac{V_{o} \times I_{o} \times \left( {V_{bus} + {n\; V_{o}}} \right) \times L_{2}}{{V_{in}^{2} \times n\; V_{o} \times L_{2}} + {V_{bus}^{2} \times L_{1} \times \left( {V_{bus} + {n\; V_{o}} - V_{in}} \right)}}}} & (4)\end{matrix}$

As can be seen from formula (4), as DC voltage V_(bus) is relativelylarge, the next multinomial can be approximated as a constant. The peakvalue of input current I_(in) can thus vary approximately with thevariation of sine half-wave DC input voltage V_(in), in order to achievepower factor correction. It should be noted that the above derivationsof various formulas may be suitable for the derived result when theexcitation inductance current of transformer T₁ operates in BCM. Ofcourse, transformer T₁ may operate in other modes, and other formulasand/or derivations may apply thereto.

As can be seen from the above calculation procedure, in an AC-DC powerconverter of particular embodiments, for two-stage power stage circuits,only one transistor and one control and driving circuit may be utilisedfor energy transmission. In addition, power factor correction and outputof a substantially constant electrical signal to power a load can alsobe achieved. When particular embodiments operate in a second operationmode, because energy of both the first energy storage element (e.g.,inductor L₁) and the second energy storage element (e.g., transformerT₁) can be released to the load, the voltage-withstanding or breakdownrequirement for the third energy storage element (e.g., capacitor C₂)may be relatively low. In addition, particular embodiments utilizerelatively simple but high accuracy control, with relatively smallripples and good overall stability, and thus are particularly suitablefor the driving of LED type loads.

Referring now to FIG. 4A, shown is a schematic block diagram of a secondexample AC-DC power converter in accordance with embodiments of thepresent invention. In this particular example, the first energy storageelement of the AC-DC power converter is inductor L₂, the second energystorage element is transformer T₁ and the third energy storage elementis capacitor C₄.

Referring now to FIG. 4B, shown is a conductive path diagram for theAC-DC power converter of FIG. 4A in the first operation mode. When inthe first operation mode, control and driving circuit 401 can controltransistor Q to turn on, and diode D₁, inductor L₂, andtransistor/switch Q can form the first conductive path (denoted by anencircled “1”). The sine half-wave DC input voltage V_(in) can storeenergy in the inductor L₂ through the first conductive path, and thencurrent I₁ of the inductor L₂ may rise (e.g., continually) in the firstconductive path. Also during the first operation mode, primary sidewindings L_(p) of transformer T₁, diode D₄, and capacitor C₄ can form asecond conductive path (denoted by an encircled “2”), and DC voltageV_(bus) across capacitor C₄ can store energy in transformer T₁ throughthe second conductive path. Also, the second current flowing throughprimary side windings L_(p) may rise (e.g., continually) as part of thesecond conductive path.

Referring now to FIG. 4C, shown is a conductive path diagram for theAC-DC power converter of FIG. 4A when in the second operation mode. Inthis mode, control and driving circuit 401 can control transistor Q toturn off, and inductor L₂, transformer T₁, capacitor C₄, and diode D₃can form a third conductive path (denoted by an encircled “3”). As partof the third conductive path, inductor L₂ may release energy, and thefirst current flowing through inductor L₂ can decline (e.g.,continually).

A portion of the energy of inductor L₂ may be transferred to the loadthrough transformer T₁, and a remaining portion of the energy ofinductor L₂ may be for charging capacitor C₄, and DC voltage V_(bus) canbe generated across capacitor C₄. When the capacitance of capacitor C₄is relatively large, DC voltage V_(bus) may be nearly constant. Also,secondary side windings L_(s) of transformer T₁, diode D₂, and capacitorC₃ form a fourth conductive path (denoted by an encircled “4”), andenergy stored in transformer T₁ may be transferred to the load via thefourth conductive path.

As can be seen from the above, the first and third conductive paths ofthis example can form a boost-buck power stage circuit. The boost-buckpower stage circuit can convert the sine half-wave DC input voltageV_(in) into a substantially constant DC voltage V_(bus) across capacitorC₄. The second and fourth conductive paths can form a flyback powerstage circuit. The flyback power stage circuit can receive DC voltageV_(bus), and may generate a substantially constant output voltage V_(o)and a substantially constant output current I_(o) to drive the LED load.For example, diode D₃ can be used to provide a continuing current flowpath for inductor L₂ for the third conductive path. Also, diode D₄ maybe used to prevent the input voltage from having a discharge path toground.

Referring now to FIG. 5A, shown is a schematic block diagram of a thirdexample AC-DC power converter in accordance with embodiments of thepresent invention. In this particular example, the first energy storageelement of AC-DC power converter in present embodiment is inductor L₃,the second energy storage element is inductor L₄, and the third energystorage element is capacitor C₅.

Referring now to FIG. 5B, shown is a conductive path diagram for theAC-DC power converter of FIG. 5A when in the first operation mode. Inthis mode, control and driving circuit 501 can control transistor Q toturn on. Also, diode D_(I), inductor L₃, capacitor C₃, and switch Q canform a first conductive path (denoted by an encircled “1”). The sinehalf-wave DC input voltage V_(in) can store energy in inductor L₃through the first conductive path, and current I₁ of the inductor T₁ mayrise (e.g., continually) as part of the first conductive path.

The sine half-wave DC input voltage may transfer energy to the loadthrough the first conductive path. Also in the first operation mode,capacitor C₅, inductor L₄, capacitor C₃, and switch Q can form a secondconductive path (denoted by an encircled “2”). For the second conductivepath, capacitor C₅ may release energy, and inductor L₄ can store energy.The current of inductor L₄ can rise, and the energy of capacitor C₅ maybe provided to the load via the second conductive path.

Referring now to FIG. 5C, shown is a conductive path diagram for theAC-DC power converter of FIG. 5A when in the second operation mode. Inthis mode, control and driving circuit 501 can control transistor Q tobe off. Inductor L₃, capacitor C₃, diode D₅, and capacitor C₅ can form athird conductive path (denoted by an encircled “3”), and current I₁ ofinductor L₃ can decline (e.g., continually). Inductor L₃ may releaseenergy via the third conductive path, and a portion of its energy can beprovided to the load, while a remaining portion of the energy frominductor L₃ can be provided for charging capacitor C₅. Also, DC voltageV_(bus) may be generated across capacitor C₅. Also during the secondoperation mode, inductor L₄, capacitor C₃, and diode D₅ can form afourth conductive path (denoted by an encircled “4”), and inductor L₄may transfer energy to the load via the fourth conductive path.

Diode D₅ may be used as a continuing current flow path of inductor L₃and L₄. In this particular example, the first and third conductive pathsmay form a boost-buck power stage circuit. The boost-buck power stagecircuit can convert sine half-wave DC input voltage V_(in) into asubstantially constant DC voltage V_(bus) across capacitor C₅. Also, thesecond and fourth conductive paths can form a buck power stage circuit.The buck power stage circuit can receive DC voltage V_(bus), and maygenerate a substantially constant output voltage V_(o) and asubstantially constant output current I_(o) to drive the load (e.g., anLED load).

The following will describe power factor correction principles of theAC-DC power converter of particular embodiments, as well as thesubstantially constant outputs under different operation modes.According to operating principles of the buck power stage circuit, whencurrent of inductor L₃ operates in a discontinuous conduction mode (DCM)and inductor L₄ operates in BCM, output current I_(o) can be obtained byformula (5).

$\begin{matrix}{I_{o} = {{\frac{I_{{pk}\; a}}{2} \times \frac{T_{on} + T_{{off}\; a}}{T_{S}}} + {\frac{I_{{pk}\; 4}}{2} \times \frac{T_{on} + T_{{off}\; 4}}{T_{S}}}}} & (5)\end{matrix}$

For example, I_(pk3) can denote a peak value of the current of inductorL₃, and I_(pk4) can denote a peak value of the current of inductor L₄.Also, t_(off3) can denote the time during which the current of inductorL₃ drops to zero from its peak value, and t_(off4) can denote the timeduring which the current of inductor L₃ drops to zero from its peakvalue. Further, t_(s) may denote a switching period (e.g., the sum oft_(on) and t_(off4)). For example, the peak value of the current ofinductor L₃ can be obtained as below by formula (5.1).

$\begin{matrix}{I_{{pk}\; 3} = {\frac{V_{in} - V_{o}}{L_{3}} \times t_{on}}} & (5.1)\end{matrix}$

Here, V_(in) can denote the sine half-wave DC input voltage, V_(o) candenote the output voltage, L₃ can denote the inductance value ofinductor L₃, and t_(on) can denote the conduction time of switch Q. Thepeak current of inductor L₄ can be obtained as below by formula (5.2).

$\begin{matrix}{I_{{pk}\; 4} = {\frac{V_{bus} - V_{o}}{L_{4}} \times t_{on}}} & (5.2)\end{matrix}$

Here, V_(bus) can denote DC voltage V_(bus) across capacitor C₅, and L₄can denote the inductance value of inductor L₄. In addition, timet_(off3) during which the current of inductor L₃ drops to zero from itspeak value can be obtained as below by formula (5.3).

$\begin{matrix}{t_{{off}\; 3} = {\frac{V_{in} - V_{o}}{V_{bus} + V_{o} - V_{in}} \times t_{on}}} & (5.3)\end{matrix}$

Time t_(off4) during which current of inductor L₄ drops to zero from itspeak value can be obtained by formula (5.4).

$\begin{matrix}{t_{{off}\; 4} = {\frac{V_{bus} - V_{o}}{V_{o}} \times t_{on}}} & (5.4)\end{matrix}$

A switching period t_(s) of transistor Q can be determined as below byformula (5.5).

$\begin{matrix}{t_{S} = {{t_{on} + t_{{off}\; 4}} = {\frac{V_{bus}}{V_{o}} \times t_{on}}}} & (5.5)\end{matrix}$

By rearranging formulas of I_(pk3), I_(pk4), t_(off3), t_(off4) andt_(s) into formula (5), output current I_(o) can be determined as belowper formula (6).

$\begin{matrix}{I_{o} = {\frac{t_{on}}{2\; V_{bus}} \times \left\lbrack {\frac{\left( {V_{in} - V_{o}} \right)^{2} \times V_{o}}{\left( {V_{bus} + V_{o} - V_{in}} \right) \times L_{2}} + \frac{\left( {V_{bus} - V_{o}} \right)^{2}}{L_{4}}} \right\rbrack}} & (6)\end{matrix}$

As can be seen formula (6), in order to make output current I_(o)substantially constant, only conduction time t_(on) may be controlled tomake the product of conduction time t_(on) and the following polynomialto be a constant value. Similarly, in this particular example, controland driving circuit 501 can adjust conduction time t_(on) to controloutput current I_(o) to be substantially constant by primary sidecontrol according to peak value I_(pk3) of the first current of inductorL₃, peak value I_(pk3) of the current of inductor L₄, time t_(off3)during which current of inductor L₃ drops to zero from its peak value,and time t_(off4) during which current of inductor L₄ drops to zero fromits peak value.

According to the operating principles of a boost power stage circuit,input current I_(in) of the AC-DC power converter (the first current I₁of the third inductor L₃) can be obtained by the following formula (7).

$\begin{matrix}{I_{1} = {\frac{I_{{pk}\; 3}}{2} \times \frac{t_{on} + t_{{off}\; 3}}{t_{S}}}} & (7)\end{matrix}$

For example, t_(on) can be obtained from the above formula (6), as shownbelow in formula (7.1).

$\begin{matrix}{t_{on} = {2 \times I_{o} \times V_{bus} \times \frac{\left( {V_{bus} + V_{o} - V_{in}} \right) \times L_{3} \times L_{4}}{\begin{matrix}{{\left( {V_{in} - V_{o}} \right)^{2} \times V_{o} \times L_{4}} + {\left( {V_{bus} - V_{o}} \right)^{2} \times L_{3} \times}} \\\left( {V_{bus} + V_{o} - V_{in}} \right)\end{matrix}}}} & (7.1)\end{matrix}$

By rearranging the computational formulas of I_(pk3), t_(on), t_(off3)and t_(S) into formula (7), the input current can be determined as belowper formula (8).

$\begin{matrix}{I_{in} = {V_{in} \times \frac{V_{o} \times I_{o} \times V_{bus} \times L_{4}}{{\left( {V_{in} - V_{o}} \right)^{2} \times V_{o} \times L_{4}} + {\left( {V_{bus} - V_{o}} \right)^{2} \times L_{3} \times \left( {V_{bus} + V_{o} - V_{in}} \right)}}}} & (8)\end{matrix}$

From formula (8), it is clear that as DC voltage V_(bus) is relativelylarge, the next multinomial can be approximated as a constant. Thus, theinput current I_(in) can vary approximately along with variation of thesine half-wave DC input voltage V_(in), so as to realize power factorcorrection. As can be seen from this example, only one transistor andone control and driving circuit may be utilized to satisfy the circuitdriving requirements. Also, power factor correction and output of asubstantially constant signal can be achieved. Moreover, thevoltage-withstanding requirement of the third energy storage element(e.g., capacitor C₅) may be relatively small, further reducing overallcosts.

Referring now to FIG. 6A, shows is a schematic block diagram of a fourthexample AC-DC power converter in accordance with embodiments of thepresent invention. In this particular example, the first energy storageelement of the AC-DC power converter is transformer T₁, the secondenergy storage element is inductor L₅, and the third energy storageelement is capacitor C₆.

Referring now to FIG. 6B, shown is a conducing path diagram for theAC-DC power converter of FIG. 6A when in the first operation mode. Inthis mode, the control and driving circuit 601 can control transistor Qto turn on. Also, diode D₁, primary side windings L_(P) of transformerT₁, and transistor Q can form a first conductive path (denoted by anencircled “1”). In the first operation mode, sine half-wave DC inputvoltage can store energy in transformer T₁ via the first conductivepath, and current I₁ of the primary side windings of transformer T₁ canrise (e.g., continually).

Also during the first operation mode, capacitor C₆, inductor L₅,capacitor C₃, diode D₆, and transistor Q can form a second conductivepath (denoted by an encircled “2”). Via the second conductive path,capacitor C₆ can release energy, and inductor L₅ can store energy. Also,current I₂ of inductor L₅ can rise, and energy stored in capacitor C₆may also be provided to the load.

Referring now to FIG. 6C, shown is a conductive path diagram for theAC-DC power converter of FIG. 6A when in the second operation mode. Inthis mode, control and driving circuit 601 can control transistor Q toturn off. The secondary side windings of transformer T₁, diode D₂,capacitor C₃, diode D₇, and capacitor C₆ may form a third conductivepath (denoted by an encircled “3”). Via the third conductive path,transformer T₁ may release energy, and current I_(I) of transformer T₁can decline (e.g., continually). A portion of the energy transformer T₁may be provided to the load, while a remaining portion of the energyfrom transformer T₁ may be for charging capacitor C₆ to generate DCvoltage V_(bus).

Also in the second operation mode, in Dr. L₅, capacitor C₃, and diode D₇may form a fourth conductive path (denoted by an encircled “4”).Inductor L₅ may transfer energy to the load via the fourth conductivepath. Here, the first conductive and third conductive paths may form aflyback power stage circuit. The flyback power stage circuit can receivesine half-wave DC input voltage V_(in), and may generate a substantiallyconstant DC voltage V_(bus) across capacitor C₆. Also, the second andfourth conductive paths may form a buck power stage circuit. The buckpower stage circuit can receive V_(bus) across capacitor C₆, and maygenerate via the fourth conductive path, a substantially constant outputvoltage V_(o) and a substantially constant output current I_(o) to drivethe load (e.g., an LED). In the example AC-DC power converter of FIG.6A, the first and second conductive paths may share transistor Q andcontrol and driving circuit 601.

In particular embodiments, an AC-DC power converter may satisfy circuitdriving requirements with a transistor and a control and drivingcircuit, by using two-stage power stage circuits. Power factorcorrection can be achieved, and a substantially constant electricalsignal (e.g., voltage, current) can be provided at the output. Thisapproach can provide relatively high control accuracy, small ripples,and steady output signals. Also, the voltage-withstanding or breakdownrequirement of the third energy storage element (e.g., capacitor C₆) maybe relatively small, and thus the overall costs can be reduced.

Those skilled in the art will recognize that other techniques orstructures, as well as circuit layout, arrangement, components, etc.,can be applied to the described embodiments. For example, the firststage power stage circuit of the may be used to realize a power factorcorrection function, while the second stage power stage circuit can beused to realize substantially constant control for the output electricalsignal (e.g., voltage, current). In addition, the power stage circuitrycan include any suitable topology (e.g., boost, buck, boost-buck,flyback, forward, etc.). As such, the conductive paths as describedherein may vary, such as including additional or different components,based on the given power stage topology.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with modifications as are suited to the particularuse contemplated. It is intended that the scope of the invention bedefined by the claims appended hereto and their equivalents.

What is claimed is:
 1. An AC-DC power converter, comprising: a) arectifier bridge and filter configured to convert an external AC voltageto a sine half-wave DC input voltage; b) a first energy storage elementconfigured to store energy from said sine half-wave DC input voltage viaa first current through a first conductive path when in a firstoperation mode, wherein said first current rises during said firstoperation mode; c) a second energy storage element configured to storeenergy from a second DC voltage via a second current through a secondconductive path when in said first operation mode, wherein said secondcurrent rises during said first operation mode; d) a transistorconfigured to share said first and second conductive paths; e) saidfirst energy storage element being configured to release energy to athird energy storage element and a load through a third conductive pathwhen in a second operation mode, wherein said second DC voltage isconfigured to be generated on said third energy storage element, andwherein said first current declines during said second operation mode;and f) said second energy storage element being configured to releaseenergy to said load through a fourth conductive path during said secondoperation mode, wherein a peak value of said first current is configuredto vary along with said sine half-wave DC input voltage, and an outputof said AC-DC converter is configured to be substantially constant. 2.The AC-DC power converter of claim 1, wherein said sine half-wave DCinput voltage is configured to release energy to said load through saidfirst conductive path in said first operation mode.
 3. The AC-DC powerconverter of claim 2, wherein said second DC voltage is configured torelease energy to said load through said second conductive path.
 4. TheAC-DC power converter of claim 1, wherein: a) said first energy storageelement comprises a first inductor; b) said second energy storageelement comprises a second inductor; and c) said third energy storageelement comprises a capacitor.
 5. The AC-DC power converter of claim 1,further comprising a control and driving circuit is configured toreceive peak current signals of said first and currents, and to generatea driving signal to drive said transistor.
 6. The AC-DC power converterof claim 1, wherein: a) said transistor is configured to be on duringsaid first operation mode; and b) said transistor is configured to beoff during said second operation mode.
 7. The AC-DC power converter ofclaim 1, wherein a first power stage circuit comprises said first andthird energy storage elements, and said first and third conductivepaths.
 8. The AC-DC power converter of claim 1, wherein a second powerstage circuit comprises said second energy storage element, and saidsecond and fourth conductive paths.
 9. The AC-DC power converter ofclaim 1, wherein said output of said AC-DC power converter comprises asubstantially constant output current configured to drive alight-emitting diode (LED) load.
 10. The AC-DC power converter of claim1, wherein said first conductive path is configured to be formed duringeach switching cycle of said AC-DC power converter.
 11. The AC-DC powerconverter of claim 1, wherein a switching cycle of said AC-DC powerconverter comprises said first and second operation modes.